The present invention relates to electronic devices having heterojunction interfaces. A heterostructure or heterojunction interface or refers to a region or interface that occurs at the junction between two different semiconductor crystal materials. The present invention is directed to heterostructure devices such as, for example, varactors. Varactors, as defined in Wikipedia (also referred to as varicap diodes, varactor diodes, variable capacitance diodes, variable reactance diodes or tuning diodes) are a type of diode which have a variable capacitance that is a function of the voltage impressed on their terminals.
U.S. Pat. No. 5,422,613 to Nativ, hereby incorporated by reference, discloses an integrated varactor diode frequency multiplier assembly. Another example of a varactor is described in the publication by Tanuma, N, “Capacitance Analysis of Al0.25Ga0.75N/GaN Heterostructure Barrier Varactor Diodes,” Phys. Stat. Sol. (c) 2, No. 7, 2692-2695 (2005) (hereinafter Tanuma) (hereby incorporated by reference). The varactor of Tanuma appears to require a voltage bias as explained in Section 2 of Tanuma where it states:                The calculated C-V and I-V characteristics in the presence and absence of the piezoelectric field are compared in FIG. 1. The C-V characteristics shift to a lower bias voltage due to the piezoelectric field induced by both the spontaneous polarization in GaN and AlGaN and the stress-induced field within the barrier.Tanuma shows as the solid line in FIG. 1, the CV curve of a GaN/AlGaN varactor. The solid line reveals a shift away from V=0 due to the polarization charges induced at the surfaces of the AlGaN layer.        
In the following the terminology Group 13 (IUPAC) and Group III refer to the same group of elements. The Group 13 group comprises boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl), and ununtrium (Uut); each of which has three electrons in their outer valence levels.